Many different approaches have been made to the problem of monitoring and indicating the discharge condition of electric storage batteries, especially in applications where the battery is first charged and then is used in the discharge mode for a considerable period of time, such as in a mobile vehicle, before it is again returned to the charger to be re-charged.
One valuable approach to this problem is to use an ampere-hour meter. Very accurate results have been obtained with such meters. However, accuracy is sometimes limited by the fact that the number of ampere hours obtainable from a battery depends very much upon the rate of discharge, fewer ampere hours being available when the battery is rapidly discharged. Furthermore, the ampere-hour meter approach requires insertion in the circuit of some means for measuring current, such as a current measuring shunt. That is not nearly so convenient as simply measuring the battery terminal voltage.
Various battery terminal voltage measurement systems have been employed for the purpose of monitoring battery discharge, with varying degrees of success. The present invention is in this category.
One voltage measurement technique for monitoring battery discharge consists of simply measuring the open circuit battery voltage. The stabilized open circuit battery voltage is a very accurate indication of the discharge state of the battery. However, it may take hours for the open circuit battery voltage to stabilize after each discharge interval, so that the stabilized reading is not available very much of the time. Furthermore, no open circuit battery voltage is available during a loaded condition of the battery.
Another approach is simply to measure and read the instantaneous battery terminal voltage on a continuing basis. However, this requires interpretation, since the terminal voltage varies during operation of the apparatus powered by the battery, having a depressed value during loading, dependent upon the magnitude of the load.
Various unidirectional systems have been devised for detecting and registering downward excursions in battery voltage under load, as an indication of the discharge condition of the battery. Some of these have operated on the basis of recognition of a sustained under-voltage condition before any registration is made, and others are really under-voltage analyzers which recognize and register under-voltage conditions in a setting recognizing previously registered under-voltage conditions. Very accurate results have been obtained with such systems, especially when applied to apparatus having known patterns of loading, even where the loading is quite discontinuous, such as in the operation of industrial forklift trucks. Examples of successful systems of this sort are disclosed in a prior U.S. Pat. No. 4,193,026 issued to Eugene P. Finger and Eugene A. Sands on Mar. 11, 1980 and assigned to the same assignee as the present application.
The present invention is an improvement upon the systems disclosed in the above mentioned U.S. Pat. No. 4,193,026. The present invention is most closely related to the system illustrated in FIG. 1 of that prior patent.
There are a number of separate objects of the present invention which may be separately listed as follows:
A. Greater economy of construction.
B. Greater economy and reliability of operation.
C. Improved accuracy of measurement.
D. Improved ease of calibration adjustment.
E. Automatic reset to the fully charged indication after charging, even when the battery has not been disconnected for charging.
F. More rapid and reliable testing of the system.
G. Provision of an improved visual indicator of charge condition.
Many other objects and advantages of the invention will be apparent from the following description and the accompanying drawings.